Slotted beam piezoelectric composite

ABSTRACT

One aspect of the present patent application is an energy harvesting device comprising a composite structure including a base spring and a piezoelectric structure. The base spring has a base spring surface having elevated portions separated by a recessed portion. The piezoelectric structure substantially crosses the recessed portion. In one aspect the piezoelectric structure includes a piezoelectric element that is bonded to the elevated portions. In another aspect, the base spring has a base spring stiffness. The piezoelectric element has a piezoelectric element stiffness. The base spring stiffness is less than the piezoelectric element stiffness. In another aspect, the composite structure has a natural frequency of vibration, and this natural frequency of vibration of the composite structure is automatically adjustable. In another aspect, the piezoelectric elements are stacked. In another aspect, the piezoelectric structure is located in the recessed portion.

RELATED APPLICATIONS

This application claims the benefit of provisional patent application60/739,976, filed Nov. 23, 2005 incorporated herein by reference.

This application also incorporates by reference the material inprovisional patent application 60/753,679, filed Dec. 22, 2005.

This invention was made with Government support under contract numberN00024-04-C-4116, awarded by the US Department of the Navy. TheGovernment has certain rights in the invention.

FIELD

This patent application generally relates to energy harvesting. Moreparticularly, it relates to a system for obtaining energy from anambient vibrating source. Even more particularly, it relates to animproved cantilever beam spring for harvesting energy with piezoelectricelements.

BACKGROUND

Harvesting energy from a vibrating structure, such as a vehicle, hasinvolved mounting piezoelectric (PZT) elements along with a proof massto provide a vibrating beam. Previous investigators found advantage inproviding PZT elements so strain is uniform across the area of each PZTelement as the cantilever beam spring vibrates. If some areas generatemore current than other areas the current from one area is shunted tothe other area and some of the power is dissipated. Thus, investigatorshave looked for schemes that provide uniform strain with a high value.

It is well known that rectangular cantilever beam springs have morestrain at the end near the base and less strain near the proof mass. Itis possible to provide more uniform strain in bending by tapering thebeam in width or in thickness, as shown in in Marks' Standard Handbookfor Mechanical Engineers, Eighth Edition, McGraw Hill, 1978, pages.Various tapers are shown including a parabolic curvature, as shown inFIG. 1, and a linear taper.

A book, Energy Scavenging for Wireless Sensor Networks, by Roundy, etal., Kluwer Academic Publishers, 2004, suggested “varying the width ofthe beam such that the strain along the length of the beam is the sameas the strain at the fixed end, resulting in a larger average strain” toimprove power obtained from a piezoelectric mounted on the beam.

However, the amount of power output by such a varying width scheme hasstill not been particularly high. Thus a better scheme is needed toprovide more power output from a vibrating beam, and these improvementsare provided in this patent application.

SUMMARY

One aspect of the present patent application is an energy harvestingdevice comprising a composite structure including a base spring and apiezoelectric structure. The base spring has a base spring surfacehaving elevated portions separated by a recessed portion. Thepiezoelectric structure substantially crosses the recessed portion.

In one embodiment the piezoelectric structure includes a piezoelectricelement bonded to the elevated portions. The base spring surface canhave a plurality of the recessed portions, wherein an elevated portionis located on either side of each the recessed portion. Thepiezoelectric element extends across each of the recessed portions andis bonded to each of the elevated portions.

In another embodiment the piezoelectric structure is located in therecessed portion. The piezoelectric structure includes a bulkpiezoelectric material, such as a single crystal piezoelectric material.A flex circuit having wiring can be used to provide contacts to thepiezoelectric structure in the recessed portion. The flex circuit canextend along an edge of the base spring. The base spring surface has anelevated portion defining a level and a portion of the bulkpiezoelectric structure can be located at that level.

Another aspect of the present patent application is an energy harvestingdevice comprising a composite structure including a base spring and apiezoelectric structure. The piezoelectric structure is bonded to thebase spring. The base spring has a base spring stiffness. Thepiezoelectric structure has a piezoelectric structure stiffness. Thebase spring stiffness is less than the piezoelectric structurestiffness.

Another aspect of the present patent application is an energy harvestingdevice comprising a composite structure including a base spring, apiezoelectric structure, and a proof mass. Natural frequency ofvibration of the composite structure is automatically adjustable.

Another aspect of the present patent application is an energy harvestingdevice comprising a composite structure including a base spring and apiezoelectric structure. The base spring has a base spring effectivethickness. The piezoelectric structure has a piezoelectric structureeffective thickness. The piezoelectric structure effective thickness isgreater than the base spring effective thickness.

In one embodiment, the base spring has a first side and a second sideopposite the first side. The piezoelectric structure includes a firstpiezoelectric element mounted on the first side and a secondpiezoelectric element mounted on the second side. The piezoelectricstructure effective thickness is about equal to a spacing between thefirst piezoelectric element and the second piezoelectric element.

Another aspect of the present patent application is an energy harvestingdevice comprising a composite structure including a base structure andstacked piezoelectric elements, wherein the stacked piezoelectricelements are mounted to the base structure.

One embodiment also includes a plurality of rectifier bridges, whereineach of the stacked piezoelectric elements is connected to a differentone of the rectifier bridges.

Another aspect of the present patent application is an energy harvestingdevice comprising a composite structure including a base spring and afirst piezoelectric element, wherein the base spring includes a firstside wherein the base spring has a base spring maximum thickness,wherein the first piezoelectric element has a first piezoelectricelement thickness, wherein the base spring maximum thickness is greaterthan the first piezoelectric element thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following detailed description,as illustrated in the accompanying drawings, in which:

FIG. 1 is a cross sectional view of a tapered cantilever beam springmounted to a structure, the beam spring having a proof mass and bondedpiezoelectric elements;

FIG. 2 is a cross sectional view of a cantilever beam spring similar tothe beam spring of FIG. 1 but with a rectangular cross section;

FIG. 3 is a cross sectional view of the cantilever beam spring of FIG. 2but with the piezoelectric elements removed;

FIG. 4 is a cross sectional view of one embodiment of a cantilever beamspring of the present patent application in which slots are provided inthe beam spring having a proof mass, and piezoelectric elements arebonded to remaining surfaces on either side of the slots;

FIG. 5 is a cross sectional view of an embodiment of a cantilever beamspring of the present patent application in which slots are provided ina tapered beam spring having a proof mass, and piezoelectric elementsare bonded to remaining surfaces on either side of the slots;

FIG. 6 and DETAIL A are cross sectional views of an embodiment of acantilever beam spring similar to that of FIG. 5 in which slots areprovided in a tapered beam spring having a proof mass, and a doublelayer of piezoelectric elements are bonded to remaining surfaces oneither side of the slots;

FIG. 7 is a cross sectional view of another tapered embodiment of acantilever beam spring of the present patent application in which alarge slot is provided in the beam spring which is filled with amaterial such as foam and piezoelectric elements are bonded to thematerial and to remaining surfaces on either side of the slot;

FIG. 8 a is a top view of a slotted tapered cantilever beam spring ofthe present patent application;

FIG. 8 b is a side view of the slotted tapered cantilever beam spring ofFIG. 8 a;

FIG. 8 c is a three dimensional view of the slotted tapered cantileverbeam spring of FIG. 8 a;

FIG. 8 d is an enlarged side view of one of the slots in the slottedtapered cantilever beam spring of FIG. 8 a;

FIG. 9 is a cross sectional view of a tapered embodiment of a cantileverbeam spring of the present patent application in which slots areprovided in the beam spring and piezoelectric elements are bonded toremaining surfaces on either side of the slots and in which the resonantfrequency of the cantilever beam and the proof mass can be tuned byadjusting compression of springs;

FIGS. 10 a and 10 b are a cross sectional views of a tapered embodimentof a cantilever beam spring of the present patent application in whichslots are provided in the beam spring and piezoelectric elements arebonded to remaining surfaces on either side of the slots and in whichthe resonant frequency of the cantilever beam and the proof mass can betuned by adjusting location of magnets;

FIGS. 11 a and 11 b are a cross sectional views of a tapered embodimentof a cantilever beam spring of the present patent application in whichslots are provided in the beam spring and piezoelectric elements arebonded to remaining surfaces on either side of the slots and in whichthe location of the proof mass can be tuned with a motor;

FIG. 12 is a block diagram showing a circuit for using energy harvestedby the energy harvesting piezoelectric elements to power the motor ofFIGS. 11 a and 11 b.

FIG. 13 a is a top view of a slotted tapered cantilever beam spring ofanother embodiment of the present patent application having a bulk PZTelement in each slot;

FIG. 13 b is a side view of the slotted tapered cantilever beam springof FIG. 13 a showing a bulk PZT element in each slot;

FIG. 13 c is a three dimensional view of the slotted tapered cantileverbeam spring of FIG. 13 a;

FIG. 13 d is an enlarged side view of two of the slots in the slottedtapered cantilever beam spring of FIG. 13 a with a bulk PZT element ineach slot;

FIG. 14 a is a block diagram showing how PZT elements, such as stackedPZT elements, can be arranged in series with each other and with a dioderectification bridge;

FIG. 14 b is a block diagram showing how PZT elements, such as stackedPZT elements, can be arranged in parallel with each other and with adiode rectification bridge;

FIG. 14 c is a block diagram showing an arrangement of PZT elements,such as stacked PZT elements, each connected to its own dioderectification bridge; and

FIG. 14 d is a cross sectional view of a beam with stacked PZT elementsarranged in series with each other and with a diode rectification bridgeas shown in FIG. 14 a.

DETAILED DESCRIPTION

The present patent application provides an improved cantilever beamspring that substantially increases the power output beyond thatavailable from a standard cantilever beam spring with varying width, asdescribed in the book by Roundy. The present application also providesimproved tuning to more nearly match the natural frequency of vibrationof the cantilever beam spring to the vibration frequency of thestructure to which it is mounted to more efficiently harvest energy fromthat vibration.

Harvesting energy from a vibrating structure can be accomplished bybonding piezoelectric (PZT) elements 20 a, 20 b on surfaces 21 a, 21 bof tapered cantilever beam spring 22 t and mounting proof mass 24, asshown in FIG. 1. Cantilever beam spring 22 t is connected to base 26that is attached to vibrating structure 28 with threaded bolts 29 a.Cantilever beam spring 22 t and base 26 can be fabricated in one unitarystructure, and fillets 29 b can be provided there between. If cantileverbeam spring 22 t vibrates in the plane of the paper the strain ishighest at its top and bottom surfaces. Cantilever beam spring 22 tvibrates because of its contact with vibrating structure 28. PZTelements 20 a, 20 b move with surfaces 21 a, 21 b of cantilever beamspring 22 t. PZT elements 20 a, 20 b are alternately strained in tensileand compressive strains, and they generate an alternating current thatdepends on the magnitude of strain rate they experience. Thisalternating current can then be immediately used or it can be stored,for example in a rechargeable battery. The entire device may beprotected in enclosure 30.

Because the ideas of the present patent application apply regardless oftaper, and because taper adds complexity to the description of theseideas, in the calculations below we will include a thick beam withouttaper and provide results for beam 22 r with rectangular cross section,as shown in FIG. 2. Providing taper improves upon these results but thegeneral principles obtained still apply.

As described above, the amount of electricity generated by PZT elements20 a, 20 b increases with the strain they experience. The amount ofstrain that is generated at each surface 21 a, 21 b of beam 22 r towhich PZT elements 20 a, 20 b are bonded and on which they ride, dependson the width W (the dimension in FIG. 2 into the paper) and thethickness H of beam 22 r and on the lateral displacement x of beam 22 rfrom its neutral position along axis 32.

First, considering beam 22 r by itself, without PZT elements 20 a, 20 b,as shown in FIG. 3, when beam 22 r is deflected a fixed amount, x, theelastic energy U provided to beam 22 r, is given by

$U = {\frac{1}{2}K\; x^{2}}$where K, the spring constant of beam 22 r, is a measure of the stiffnessof beam 22 r. K depends on material properties E of beam 22 r, such asYoung's modulus, and geometrical factors of beam 22 r, including lengthL and moment of inertia I.

$K = \frac{E\; I}{L}$

Moment of inertia I depends linearly on width W and as the third powerof thickness H of beam 22 r

$I = {\frac{1}{12}W\; H^{3}}$

Thus, choosing rectangular beam 22 r with a larger thickness H increasesmoment of inertia I and stiffness K as the third power of thickness H.For a given lateral displacement x, the energy U of vibrating beam 22 rthus also increases as the third power of the thickness H of beam 22 r.

However, as rectangular beam thickness H increases it also becomes thatmuch harder to provide the same lateral displacement x as compared toproviding that displacement to a thinner beam. Put another way, for agiven energy provided to beam 22 r, the amplitude of vibration decreasesas stiffness and spring constant K increase. Considering the equationsabove one can see that for a given energy of vibration the amplitude ofvibration decreases as thickness increases as 1/H^(3/2).

PZT elements 20 a, 20 b bonded to top and bottom surfaces 21 a, 21 b ofcomposite beam 34 r ride along with those surfaces as beam 22 rvibrates. PZT elements 20 a, 20 b acquire part of their energy ofvibration as strain is repeatedly introduced in them 20 b during thevibration of beam 22 r. The strain PZT elements 20 a, 20 b experiencedepends on the amplitude of vibration of beam 22 r so a larger amplitudeof vibration would therefore transfer more of the energy of beam 22 r toPZT elements 20 a, 20 b. Since for a given energy of vibration of beam22 r, the amplitude of vibration decreases as thickness H increases, athinner beam appears to be desirable for obtaining the higher amplitudeand imparting the greatest strain to PZT elements 20 a, 20 b. However,for a given amplitude of vibration, the energy of vibration of beam 22 rincreases as the cube of beam thickness W. With more energy of vibrationbeam 22 r has more to transfer to PZT elements 20 a, 20 b. Thus, a beamwith high thickness H and vibrating with high amplitude would bedesirable.

The present inventors realized that if they could find a way to make theseparation of PZT elements 20 a, 20 b large while making the effectivethickness of beam 22 r on which they are mounted smaller they couldprovide a larger amplitude of vibration to composite beam 34 r, a largerstrain in PZT elements 20 a, 20 b, and a larger transfer of energy frombeam 22 r to PZT elements 20 a, 20 b. Thus, the present applicantsconsidered how to both provide increasing thickness to beam 22 r tomaintain wide separation between PZT elements 20 a, 20 b mounted on beam22 r to take advantage of the power of 3 increase in PZT elementstiffness and energy harvested from PZT elements 20 a, 20 b, while atthe same time reducing effective thickness and stiffness of beam 22 r soit will vibrate with a larger amplitude and provide more strain to PZTelements 20 a, 20 b.

One embodiment provides recessed portions, such as slots 40 in beam 22r′, as shown in FIG. 4. Slots 40 include recessed portions and elevatedportions. With slots 40 the present inventors found experimentally thatthey could reduce stiffness and effective thickness H_(beam) of beam 22while maintaining separation distance H_(PZ) of PZT elements 20 a, 20 b.

Slots 40 in beam 22 r′ made beam 22 r′ flex as if it was much thinnerand much more flexible than outside dimensions H_(PZ) of adjacentelevated portions, such as ridges 42. PZT elements 20 a, 20 b mounted onridges 42 thus remained widely separated, as shown in FIG. 4. Withslotted beam 22 r′ acting mechanically as if it were a thinner beamhaving an effective thickness H_(beam) intermediate between heightH_(PZ) and slot separation distance S, and with PZT elements 20 a, 20 bretaining their wide spacing H_(PZ) because of the constraints providedby bonding to ridges 42, much more of the energy of vibration goes intoPZT elements 20 a, 20 b and much less goes into slotted beam 22 r′,increasing the energy harvested from vibrating beam 22 r′.

Slots 40 in beam 22 were machined with fillets 44 in corners 46 toreduce the chance of beam 22 cracking. PZT elements were bonded toridges 42 with Loctite Hysol E120HP epoxy 48. Epoxy 48 was deposited onboth PZT elements 20 a, 20 b and on ridges 42 of beam 22 so as to whetboth surfaces before bonding. PZT elements 20 a, 20 b were then mountedto ridges 42 of beam 22. A flat piece of rubber and a flat sheet ofaluminum (not shown) were then placed on both PZT elements 20 a, 20 band a C clamp used to clamp them solidly in place during curing. Excessepoxy that may have flowed into the slots was removed by flossing eachslot with a wire before epoxy 48 had a chance to set.

Composite beam 34 r′ can be considered to include two separate vibratingsprings, slotted beam 22 r′ and PZT spring 50 r which is formed of PZTelements 20 a, 20 b, and these two vibrating springs vibrate in parallelwith each other. PZT elements 20 a, 20 b act together as single spring50 because each PZT element 20 a, 20 b provides a restoring force to thebending of the other PZT element 20 a, 20 b. While beam 22′ bends in onedirection during its vibration, one of PZT elements 20 a, 20 b issubjected to tensile stress while the other is subjected to compressivestress, and then these roles reverse as beam 22′ bends the other wayduring its back and forth vibration.

For PZT spring 50 r formed of PZT elements 20 a, 20 b, third power ofseparation distance H_(PZ) between pair of PZT elements 20 a, 20 b ismost determinative of moment of inertia, stiffness and energy ofvibration, just as effective thickness H_(beam) of beam 22 r′ is mostdeterminative of these parameters for beam 22 r′.

The energy of vibration U imparted to composite beam 34′, including beam22 r′ and PZT spring 50 r, is shared among them as follows:U=U _(beam) +U _(PZ)where U_(beam) is the energy of beam 22 r′ and U_(PZ) is the energy inPZT spring 50 r that includes both PZT elements 20 a, 20 b acting as asingle spring element, each providing a restoring force against thebending of the other.

This equation can be rewritten as

$U = {{\frac{1}{2}K_{beam}x^{2}} + {\frac{1}{2}K_{PZ}x^{2}}}$where K_(beam) is the spring constant of beam 22 r′ and K_(PZ) is thespring constant of PZT spring 50 r, which is both PZT elements 20 a, 20b acting together as a single spring. Substituting for K, andrecognizing that by providing slots in beam 22 r′ we can providedifferent effective thicknesses for beam 22′ and for PZT spring 50 r, weget

$U = {\frac{E_{beam}W_{beam}H_{beam}^{3}x^{2}}{24\; L} + \frac{E_{PZ}W_{PZ}H_{PZ}^{3}x^{2}}{24\; L}}$where W_(beam) is the width of slotted beam 22 r′, H_(beam) is theeffective thickness of slotted beam 22 r′, W_(PZ) is the width of PZTelements 20 a, 20 b, and H_(PZ) is the separation distance between PZTelements 20 a, 20 b of PZT spring 50 r.

Now if a total energy U is transferred to slotted vibrating beam 22 r′with PZT elements 20 a, 20 b bonded to its top and bottom surfaces, andif H_(PZ) is substantially larger than the effective H_(beam) of slottedbeam 22 r′, then much more of that total energy U goes into vibration ofPZT spring 50 r and less goes into vibration of beam 22 r′, and we canexpect to increase energy harvesting from PZT elements 20 a, 20 b.

The present inventors found they could provide taper, provide a largeseparation distance of PZT elements 20 a, 20 b, and retain the same lowloss to friction of the aluminum material of beam 22 t, while reducingthe effective thickness and providing more flexibility to beam spring 22t by providing slots 40 to composite cantilever beam spring 34 t′composed of slotted tapered beam 22 t′ and PZT elements 20 a, 20 bbonded thereon that form PZT spring 50 t′, as shown in FIG. 5.

When they created tapered composite cantilever beam spring 34 t′ withslotted tapered beam 22 t′ and with PZT elements 20 a, 20 b bonded toridges 42 and spanning across slots 40 they found that they stilldeveloped a relatively stiff composite structure, but now the majorityof the resistance to bending was provided by PZT spring 50 t′ ratherthan by slotted beam 22 t′ on which PZT elements 20 a, 20 b weremounted. The present inventors found that this configuration reduced themoment of inertia of beam 22 t′ compared to that of tapered beam 22 ofFIG. 1 and compared to the moment of inertia of PZT spring 50 t′.

Under comparable vibration conditions, tapered composite cantilever beamspring 34 t′ also develops a lot more strain in PZT elements 20 a, 20 bthan the method of providing PZT elements 20 a, 20 b along smoothsurfaces 21 a, 21 b of smooth beam spring 22 shown in FIG. 1. Thepresent inventors found that this slotted composite configuration workswell to provide substantially more strain, and substantially moreelectrical power from PZT elements 20 a, 20 b than cantilever beamspring 22 that is not slotted. Because slotted beam 22 t′ is stilltapered, a uniform distribution of strain from one end of beam 22 t′ tothe other continues to be expected.

Typically a metal such as steel, aluminum, beryllium copper, titanium,or superelastic nickel titanium has been used to provide the good springproperties needed for cantilever beam spring 22 because such metals canvibrate with very little loss to friction. However, until the presentpatent application, most such metals have had much more stiffness thanPZT elements 22 bonded to their surfaces. So when PZT elements 20 a, 20b were bonded to metal cantilever beam spring 22 t and when applied aload, such as proof mass 24, was applied, most of this load wassupported by the metal of cantilever beam spring 22 t and less wassupported by compression and expansion of PZT elements 20 a, 20 b.

Tapered slotted metal beam spring 22 t′ retains the full thicknessH_(PZ), from the point of view of PZT elements 20 a, 20 b. Compositestructure 34 t′ of FIG. 5 retains the stiffness contributed by PZTelements 20 a, 20 b but reduces the stiffness contributed by beam spring22 t′ because of slots 40. Thus, the present inventors providedcomposite structure 34 t′ in which the overall stiffness was dominatedby the stiffness of PZT elements 20 a, 20 b rather than by the stiffnessof underlying beam spring 22 t′. This composite structure 34 t′ alsoretained the low loss to friction provided by a smooth beam withoutslots. Most of the resistance to bending was provided by PZT elements 20a, 20 b, and hence PZT elements 20 a, 20 b experienced more strain andgenerated more energy and more electricity for harvesting than smoothbeam counterparts.

One alternative is to provide PZT elements 20 a, 20 b on a beam springfabricated of a more flexible material than a metal, such as a foam,which is much less rigid than metal, so virtually all of the resistanceto bending would be provided by PZT elements 20 a, 20 b while verylittle would be provided by the foam beam spring, even if the foam wasnot slotted. Other materials, such as rubber, wood, or a polymer such aspolyethylene can also be used. But such materials dissipate more energyto friction than metal does when it vibrates, and so some of the energyof vibration that otherwise could be converted into electrical energymay be lost. In addition foam may not be strong enough to withstand theshear forces introduced by repeated bending during vibration at largeamplitude.

Thus, a cantilever beam spring that has low losses to friction, thatprovides wide separation between PZT elements, that is highly flexible,and that can withstand the shear forces of the vibration, is desired.The present inventors found that slots 40 in aluminum cantilever beamspring 22 t′ did not add to the friction losses and that the resultingbeam spring 22 t′ retained its low dissipation and high Q factor whileretaining resistance to shearing forces, and generated substantiallymore electricity than a standard aluminum cantilever beam spring.

PZT elements 20 a 1, 20 a 2, 20 b 1, 20 b 2 can be stacked as shown inFIG. 6 and in DETAIL A to increase the amount of electricity generatedwithout enlarging slotted beam 22 t′. Insulative layer 54 is usedbetween PZT elements 20 a 1 and 20 a 2 to prevent shorting. Thin epoxyadhesive layer 56 is used to bond PZT elements 20 a 1 and 20 a 2 to beam22 t′ and to each other.

The present applicants have also designed another embodiment of a springthat provides the advantages of foam while retaining the strength of ametal beam, as shown in FIG. 7. In this embodiment, beam spring 60includes large slots 62. Low stiffness material 64, such as foam pieces,are adhesively bonded within large slots 62. PZT elements 20 a, 20 b arethen epoxied both to ridges 66 across large slots 62 and to foam pieces64 bonded within large slots 62. During vibration of beam spring 60thick areas of 68 a, 68 b of aluminum beam spring 60 on either side oflarge slots 62 and web 70 between large slots 62 withstand the shearload, retaining durability compared to a purely foam beam. Neverthelessbeam spring 60 acts as if it was almost as thin as web 70 extendingbetween large slots 62. And beam spring 60 retains most of the lowfriction of a smooth beam. Beam spring 60 therefore acts mechanically asif it is a thin beam with much less stiffness than it would have iflarge slots 62 were not machined. Stiffness of composite beam 72 isdominated by stiffness of PZT spring 74 that includes PZT elements 20 a,20 b. Because narrow slots need not be machined for this embodiment, thecost for fabricating it may be lower than for slotted beam 22 t′ of FIG.5.

In addition to foam, low stiffness material 64 can be a material, suchas rubber, wood, or a polymer, such as polyethylene or silicone. Lowstiffness material 64 is bonded in slots 62 and fills in to provide asurface to which PZT elements 20 a, 20 b can be bonded so PZT elements20 a, 20 b won't buckle. Material 64 preferably has sufficient strengthto hold PZT elements 20 a, 20 b so they bend with beam spring 60 but donot buckle.

When beam 60 bends, PZT elements 20 a, 20 b undergo tension andcompression, and the stress they experience is very high. In comparisonto that stress, the force needed to prevent PZT elements 20 a, 20 b frombuckling is very low. Low stiffness material 64 can have a stiffnessadequate to prevent buckling while having a stiffness that is about oneor two orders of magnitude lower than that of beam material 60. Thus,low stiffness material 64 can have a stiffness that is high enough toprevent PZT from buckling while being low enough so it does notcontribute to the bending stiffness of beam spring 60.

Low stiffness material 64 damascened into large slot 62 adds damping,lowering the Q of vibrating beam 22. For harvesting energy fromvibration a low loss, high Q system is preferable. So having many slotswith enough ridges to avoid buckling may be preferable to adding a lossymaterial within the slots. In consideration of the load from bending,the space D across slots 40 or the separation between ridges 66, and thethickness of PZT elements 20 a, 20 b, one can determine whether a fillermaterial in slots 40, 62 is desirable as compared to the potentiallosses to damping.

Tapered slotted beam 22 t′ has base region 80, tapered region 82, andbar region 84, as shown in FIGS. 8 a-8 d. Base region 80 is forconnection to base 26 which is attached to vibrating structure 28 (seeFIG. 5). Energy from vibrating structure 28 sets proof mass 24 ontapered beam 22 t′ vibrating so that energy can be harvested with PZTelements 20 a, 20 b which vibrate with tapered beam 22 t′ (see FIG. 5).Tapered region 82 includes slots 40 and ridges 42. Bar region 84 hasscribe lines 86 that allow ready positioning of proof mass 24 mountedthereon.

In an experiment, tapered slotted beam 22 t′ was fabricated of aluminumand had a total length of 3.438 inches and a width of 0.674 inches. Baseregion 80 was about 0.9 inches long and had a maximum height of 0.2inches. In this embodiment, bar region 84 was about 1.463 inches long,had the same width of 0.674 inches, and had the same height of 0.2inches.

Tapered region 82 tapered from a 0.2 inch height that extended 0.5inches from the left end down to a height of 0.090 inches adjacent barregion 84. Tapered region 82 includes slotted portion 88 having eightslots 40 located in this 0.687 inch long portion. In the presentexperiment the taper was linear at 3 degrees, the length of the slottedportion was 0.687 inches and PZT elements 20 a, 20 b extended 1 inch inthe length direction and extended beyond slotted portion 88 on eitherside by about 0.16 inches.

Slots 40 had a spacing D that was 0.05 inches across and were separatedby metal ridges 42 that had a dimension R that was 0.041 inches across.Remaining metal S vertically left between slots 40 was 0.03 inches high.Tapered region 82 extended past slotted portion 88 for another 0.76inches until it reached bar region 84 at rounded corners 90 having aradius of 0.063 inches. In the present experiment, eight slots wereprovided with a pitch of 0.091 inches. Beam 22 t′ was made more flexiblewhile providing multiple points of restraint to the PZT elements 20 a,20 b by making spacing D across each slot 40 small compared to overalllength L of PZT elements 20 a, 20 b (see FIG. 5) as they extended acrossslots 40 toward proof mass 24 to make tapered composite beam 34 t′.

Bonding PZT elements 20 a, 20 b to each ridge 42 prevented buckling ofPZT elements 20 a, 20 b so they maintained desired separation distanceH_(PZ) and so they acted against each other, one in tensile stress, theother in compressive stress. Bonding PZT elements 20 a, 20 b to eachmetal ridge 42 between slots 40 held each PZT element 20 a, 20 b inplace at that desired spacing. Leaving a significant area of material onridges 42 for bonding to PZT elements 20 a, 20 b provides strongintermittent support to PZT elements 20 a, 20 b to maintain the desiredseparation distance between two PZT elements 20 a, 20 b and to maintainthe taper shape provided by beam 22 t′. The constraints provided bythese solidly bonded multiple points of attachment also restricted orprevented other modes of vibration.

A larger number of thinner slots provides more points of attachment forPZT elements 20 a, 20 b so points of attachment are close enoughtogether for each PZT element spanning each slot 40 to avoid bucklingwhen that PZT element 20 a, 20 b goes into compression.

Bar region 84 is for holding proof mass 24, is about 1.4 inches long,and has scribe lines 86 that are about 0.015 inches deep forfacilitating positioning of proof mass 24. Proof mass 24 had a mass ofabout 250 grams and was connected along bar region 80 with a screw (notshown).

Variations on all these dimensions are possible. For example, a greaterwidth W increases the surface area on both sides of tapered beam 22 t′which increases the area of PZT element 20 a, 20 b that can be usedwhich increases the energy that can be harvested, as also shown by theequations above. With a wider flex beam a larger proof mass 24 may beused to retain the same mass per unit width to achieve the same naturalfrequency of vibration. Variation in the mass or its position willchange the resonant frequency of the energy harvesting composite system.

A thicker beam and deeper slots provide a greater spacing between PZTelements 20 a, 20 b while not substantially changing height H of beam 22t′, and this may also increase the energy harvested with slotted beam 22t′.

Within limits imposed by the taper, a greater length of slotted region88 of beam 22 t′ increases the area of PZT elements 20 a, 20 b extendingover slotted region 88, increasing the energy that can be harvested.

Various numbers of slots and various dimensions for each slot can beused. The present inventors expected improved results with a largernumber of narrower slots that are closer together and set the slotdimensions in the experiment determined by the ability to machinenarrowest possible slots 40 and narrowest possible ridges 42.

In the experiment, Macrofiber composite piezoelectric elements 20 a, 20b, part number M 2814 p2 obtained from Smart Material GmbH, Dresden,Germany, was bonded to each side of slotted portion 88 of tapered region82 with epoxy connecting each PZT element 20 a, 20 b to each ridge 42along tapered region 82, as shown in FIG. 5 and in FIGS. 8 a-8 c.

In one embodiment, PZT elements 20 a, 20 b are provided spanning slottedportion 88 on slotted beam 22 t′ widely separated from neutral axis 32.Beam 22 t′ was fabricated of aluminum, a material that has good springproperties so it loses little energy to friction and can vibrate for along time. Slots provided on both sides of beam 22 t′ removed about halfthe material of beam 22 t′ in the slotted region so beam 22 t′ had areduced effective thickness in this region. Beam 22 t′ had sufficientslots that, although its outer surfaces remained widely separated, itacted mechanically as if it was a narrow beam with low stiffness,providing a large amplitude of vibration for a given initial lateralforce and therefore provided a large strain to PZT elements 20 a, 20 bso they could generate more electricity.

Of course, elements 20 a, 20 b need not both be PZT material. Forexample an element replacing element 20 b can be a layer of some othermaterial that provides stiffness on that side of the composite beam tocounter movement of PZT element 20 to enable PZT element 20 to generateelectricity even though the opposed element replacing element 20 b isnot generating electricity. Two opposed PZT elements 20 a, 20 b generatetwice as much electricity as one would, however.

In the embodiment shown in FIG. 5, proof mass 24 had its center of massC located along the intersection of the extension of top and bottomtapered surfaces 21 a′. 21 b′ of cantilever beam spring 22 t′ to providea uniform strain field in PZT elements 20 a, 20 b bonded to slottedtapered surfaces 21 a′, 21 b′.

Proof mass 24 can be manually moved within a range of positions alongbar region 84 of flex beam 22 t′ to change the natural frequency of theresonant structure to provide tuning so beam 22 t′ and proof mass 24combine to provide a natural frequency of vibration that matches thevibration frequency of vibrating structure 28 on which beam 22 t′ ismounted. The present inventors recognized that for many applications,such as machines and ships that operate for long periods of time at aspecified motor speed, and provide a vibration frequency that does notchange much with time, setting up proof mass 24 manually once to providethe beam with a natural frequency matching that relatively constantvibration frequency was sufficient.

Other schemes for manually tuning can alternatively be provided. Onesuch scheme involves providing spring 100 within enclosure 102 connectedto proof mass 104, as shown in FIG. 9. With adjustment screw 106 tomanually vary compression of spring 100, and thus adjust its springconstant, the overall natural frequency of composite beam 108, thatincludes slotted tapered beam 22 t′, PZT spring 50 t′, proof mass 104,and spring 100, can be adjusted to match that of vibrating structure 28on which enclosure 102 is mounted. More than one spring 100 can beprovided.

Another scheme for manually tuning involves providing proof mass 116that includes permanent magnet 118 a within housing 102, as shown inFIGS. 10 a, 10 b. Magnet 118 b is mounted on manual adjustment screws120 so its position can be varied. As the position of magnet 118 b isvaried with adjustment screw 120 the magnetic field strength experiencedby magnet 118 a on proof mass 116 varies and so does natural frequencyof mass 116, thus providing tuning. A motor drive and a controllerconnected to adjustment screws 80 would permit automatic adjustingnatural frequency of vibration.

In addition to providing tuning, a magnet adds a non linear element tocomposite spring 50 t′ that is useful to prevent an overload condition.For example if slotted flexible beam 22 t′ vibrates with too large anamplitude, the bending strains may be sufficient to break PZT elements20 a, 20 b. Magnets 118 a, 118 b provide increasing resistance asamplitude increases. Within the normal operating range magnets 118 a,118 b, provide additional resistance that can be used to adjustfrequency of vibration. As amplitude of vibration increases, magnets 118a, 118 b provide a much greater restoring force to prevent an overloadcondition that could break PZT elements 20 a, 20 b. Thus, by combining anon-linear element, such as magnets 118 a, 118 b, with tapered slottedbeam 22 t′, if amplitude exceeds the normal zone then magnets 118 a, 118b become the major restraining force and PZT elements 20 a, 20 b are notoverloaded. While magnets 118 a, 118 b do not change the properties ofcomposite spring 50 t′, magnets 118 a, 118 b add an additional restoringforce that increases with the displacement.

For a typical slotted tapered beam 22 t′ with proof mass 24, one canexpect to vary the natural frequency by about +/−10%. This tuning rangeis likely to be sufficient for equipment that operates within such asmall range of frequencies, such as helicopters, which operate withconstant rotor speed. Increasing range of frequency can be obtained bysubstituting different slotted tapered beams 22 t′ with different width,length, or thickness.

Another scheme involves providing motor 128 connected to adjust positionof proof mass 130, as shown in FIGS. 11 a, 11 b and in the block diagramof FIG. 12. Motor 128 drives pinion gear 132 that adjusts position ofrack gear 134 connected to shaft 136 that extends through taperedslotted beam 138 with its PZT elements 139 a, 139 b and extends intomass 130. As shaft 136 is driven to the right by operation of motor 128,mass 130 is forced to the right, compressing spring 140. When shaft 136is driven to the left by operation of motor 128, tension on mass 130 isreduced and spring 140 drives mass 130 back to the left. Motor 128 mayhave an integral rotary position encoder (not shown) to provide rotaryposition feedback.

Motor 128 and pinion gear 132 are supported on mounting block 142. Shaft136 is supported by shaft supports 144 a, 144 b. Mass 130 is supportedby dovetail groove 146 in bar region 148 of beam 138.

Motor 128 may be operated with energy derived from energy harvestingprovided by PZT elements 20 a, 20 b on beam 138 and stored in capacitor160 and/or battery 162, as shown in FIG. 12. Once sufficient energy hasbeen harvested and stored, motor 128 may be operated to move mass 130gradually in one direction. The rate of energy harvesting is monitoredin feedback controller 164 which monitors voltage across capacitor 160along lines 166. If the rate of energy harvesting increases with motionprovided by motor 128, feedback controller 164 directs motor 128 tocontinue to move mass 130 in the same direction until mass 130 islocated at a position that maximizes the rate energy is harvested. Forexample, feedback controller 164 can determine a position at which therate of change of energy with changing position goes to zero. Feedbackcontroller provides direction to motor 128 along line 168 and keepstrack of parameters, such as position of mass 130, through encoder line170.

If the rate of energy harvesting decreases with motion provided by motor128, feedback controller 164 sends a signal to motor 128 along line 168to reverse direction so mass 130 moves oppositely until the position isfound that maximizes the rate energy is harvested. Feedback controller164 can be a dsPic30F6010, available from Microchip Technologies, Inc.,Chandler, Ariz.

Energy needed to operate motor 128 to move proof mass 130, and tooperate feedback controller 164, provided along line 172, may offset theenergy needed to move mass 130. For a machine whose frequency is slowlychanging the amount of additional energy generated by tuning to thatfrequency may make adding automatic control desirable.

Recognizing that vibration may be directional, the present inventorsalso found that orienting beam 22 t′ in an orientation which providesthe highest vibration levels to the system will improve the rate energymay be harvested. However, for many applications, such as machines andvehicles, the preferred orientation does not change much with time sodetermining the preferred orientation and setting up the flexing beamonce in the preferred orientation was sufficient.

In another embodiment, bulk piezoelectric elements 180, such as singlecrystal piezoelectrics, are provided in slots 40″ of slotted beam 22 t″as shown in FIGS. 13 a-13 d. Flex material 182 with electrical contacts184 a, 184 b folds into slots 40″. Wiring 186 a, 186 b connects tocontacts 184 a, 184 b on the portion of flex material 182 that bends 90degrees from slots 40″ and extends along edge 188 of slotted beam 22 t″.Flex material may be thin polyimide with conductive traces andinsulative layers built in.

In fabricating slotted beam 22 t″ bulk piezoelectric elements 180 can becooled sufficiently and/or slotted beam 22 t″ can be heated sufficientlythat bulk piezoelectric elements 180 fit into slots 40″. Upon return toroom temperature, a residual compressive stress is thereby provided tobulk piezoelectric elements 180. Alternatively, slotted beam 22 t″ canbe stretched in a pair of clamps which also opens slots 40″ to receivebulk piezoelectric elements 180. When the clamps are released a residualcompressive stress is thereby provided to bulk piezoelectric elements180. In another alternative, slotted beam 22 t″ may be subjected tobending, which will open slots 40″ on one side of slotted beam 22 t″ toreceive snuggly fitting bulk piezoelectric elements 180. Then anopposite bending load may be applied to receive snuggly fitting bulkpiezoelectric elements 180 on the opposite side of slotted beam 22 t″.Adhesive can be provided on flex material 182 or within slot 40″ toprevent the bulk piezoelectric elements from falling out. Adhesivelocated within slot 40″ should be compliant to prevent stiffening ofslotted beam 22 t″.

Bulk piezoelectric elements should exhibit a high coupling coefficient(k₃₃) along their thickness so that alternating strains generated withvibration of slotted beam 22 t″will be efficiently converted intoelectrical energy. Single crystal piezoelectric material may be obtainedfrom commercial sources for this embodiment, and these single crystalpiezoelectric materials provide high coupling coefficients. Onecommercial source for single crystal piezoelectric material is partnumber PMN-PT-28 from Morgan Electro Ceramics, Bedford, Ohio. These bulkpiezoelectric materials exhibit high coupling coefficient(k₃₃=0.86-0.90), as compared to conventional bulk piezoelectricmaterials, such as PZT-4 (k₃₃=0.70) or PZT-5H (k₃₃=0.75).

Multiple PZT elements 190 a-190 d, such as stacked PZT elements 20 a 1,20 a 2, 20 b 1, 20 b 2, as shown in FIG. 6 or such as bulk piezoelectricelements 180, as shown in FIG. 13 d, may be connected to single diodebridge 192 in series, as shown in FIG. 14 a, or in parallel, as shown inFIG. 14 b. In another connection scheme, separate diode rectificationbridge 192 a-192 d is used for each PZT element 190 a-190 d, as shown inFIG. 14 c. In this embodiment each PZT element is independentlyrectified so that if strains are not equal among PZT elements energy isnot lost from the highly strained elements into the lesser strainedelements. Furthermore, should a lead wire break from any one of the PZTelements energy will still be provided at the output.

One mechanical embodiment of the series circuit of FIG. 14 a, withstacked PZT elements 190 a-190 d mounted on beam 194 with proof mass 196and connected to single diode bridge 192, is shown in FIG. 14 d.

While the disclosed methods and systems have been shown and described inconnection with illustrated embodiments, various changes may be madetherein without departing from the spirit and scope of the invention asdefined in the appended claims.

1. An energy harvesting device, comprising a composite structureincluding a first spring and a piezoelectric structure, wherein saidfirst spring has a first spring surface having elevated portionsseparated by a recessed portion, wherein said piezoelectric structuresubstantially crosses said recessed portion, and wherein saidpiezoelectric structure includes a piezoelectric element bonded to saidelevated portions.
 2. An energy harvesting device as recited in claim 1,wherein said first spring includes a taper.
 3. An energy harvestingdevice as recited in claim 1, wherein said first spring surface has aplurality of said recessed portions, wherein an elevated portion islocated on both sides of each said recessed portion.
 4. An energyharvesting device as recited in claim 3, wherein said piezoelectricelement extends across each of said recessed portions and is bonded toeach of said elevated portions.
 5. An energy harvesting device asrecited in claim 4, wherein said first spring has a lower stiffness thansaid piezoelectric element.
 6. An energy harvesting device as recited inclaim 3, wherein said first spring includes a long axis, wherein saidrecessed portions are arranged perpendicular to said long axis.
 7. Anenergy harvesting device as recited in claim 1, wherein said compositestructure includes a plurality of said piezoelectric elements.
 8. Anenergy harvesting device as recited in claim 7, wherein said pluralityof piezoelectric elements are stacked.
 9. An energy harvesting device asrecited in claim 7, further comprising a plurality of rectifier bridges,wherein each said plurality of piezoelectric elements is connected to adifferent one of said rectifier bridges.
 10. An energy harvesting deviceas recited in claim 1, wherein one said piezoelectric element is locatedon a first side of said first spring and a second said piezoelectricelement is located on a second side of said first spring.
 11. An energyharvesting device as recited in claim 10, wherein two said piezoelectricelements are located on a first side of said first spring and two saidpiezoelectric elements are located on a second side of said firstspring.
 12. An energy harvesting device as recited in claim 11, whereinsaid two piezoelectric elements located on said first side of said firstspring are stacked and wherein said two piezoelectric elements locatedon said second side of said first spring are stacked.
 13. An energyharvesting device as recited in claim 1, wherein a low stiffnessmaterial is located within said recessed portion, wherein saidpiezoelectric element is bonded to said low stiffness material.
 14. Anenergy harvesting device as recited in claim 1, wherein said firstspring has a damping characteristic about equal to that of a firstspring without said recessed portion.
 15. An energy harvesting device asrecited in claim 1, wherein said piezoelectric structure includes asecond piezoelectric element located in said recessed portion.
 16. Anenergy harvesting device as recited in claim 15, wherein said secondpiezoelectric element includes a bulk piezoelectric material.
 17. Anenergy harvesting device as recited in claim 16, wherein said bulkpiezoelectric material includes a single crystal piezoelectric material.18. An energy harvesting device as recited in claim 16, wherein saidfirst spring surface has an elevated portion defining a level, wherein aportion of said bulk piezoelectric material is located at said level.19. An energy harvesting device as recited in claim 15, furthercomprising a flex circuit having wiring, wherein said piezoelectricstructure contacts said wiring within said recessed portion.
 20. Anenergy harvesting device as recited in claim 19, wherein said flexcircuit extends along an edge of said first spring.
 21. An energyharvesting device as recited in claim 1, wherein said first spring isfabricated of metal.
 22. An energy harvesting device as recited in claim21, wherein said metal includes a superelestic nickel titanium material.23. An energy harvesting device as recited in claim 1, wherein saidcomposite structure further includes a mass, wherein said mass ismounted on said first spring.
 24. An energy harvesting device as recitedin claim 23, wherein said mass is movable on said first spring.
 25. Anenergy harvesting device as recited in claim 23, wherein said compositestructure has a natural frequency of vibration, wherein said compositestructure is mounted on a substrate subject to vibration at a substratefrequency, wherein said substrate frequency is variable, wherein saidnatural vibration frequency of said composite structure is adjustable tomatch variation in said substrate vibration frequency.
 26. An energyharvesting device as recited in claim 23, wherein said compositestructure has a natural frequency of vibration, wherein said compositestructure further includes a second element coupled to said firstspring, wherein varying a parameter of said second element varies saidnatural frequency of vibration.
 27. An energy harvesting device asrecited in claim 26, wherein said second element includes a spring. 28.An energy harvesting device as recited in claim 26, wherein said secondelement includes a magnet.